1. Field of the Invention
The present invention relates to a phase shift measuring apparatus according to a light wave interference method utilizing optical heterodyne techniques.
More particularly, the present invention relates to a phase shift measuring apparatus capable of highly precisely measuring a gradient index, a thickness distribution, or the like of an object to be measured upon detection of a phase shift amount of a light wave passing through the object.
2. Related Background Art
Some conventional apparatuses for detecting surface precision, thicknesses and uniformity of the quality of objects to be measured comprise interferometers such as Mach-Zehnder, Fizeau, and Twyman-Green interferometers.
In a conventional interferometer of this type, a light wave passing through an object to be measured is combined with a light wave called a reference wave to detect physical information of the object on the basis of interference fringes formed by these two light sources.
In order to detect such physical information, the interference fringes are simply observed or converted into a light intensity distribution. Alternatively, a phase shift as a function of time at each position of the interference fringes is measured as in the phase interferometer.
In order to measure a gradient index of the object by using a conventional interferometer of this type, a plane wave is caused to pass through a sample having a gradient index and a phase shift of the plane wave is converted into an intensity distribution of the interference fringes, thus measuring the gradient index. A refractive index difference .DELTA.n can be given by the following equation: EQU .DELTA.n=.DELTA..phi./d
where .DELTA..phi. is the phase shift (i.e., phase difference) of the plane wave and d is the thickness of the sample.
The parameter for determining measurement precision is the phase difference .DELTA..phi.. According to a method of observing the interference fringes or a method of directly converting the interference fringes into a light intensity distribution, the phase difference .DELTA..phi. cannot be detected with high precision. Although the conventional phase interferometer can detect the phase difference .DELTA..phi. within a tolerance, the light wave phase shift as a function of time requires fine vibration of a mirror or the like with high precision. Therefore, the structure of the phase interferometer is complicated and undesirably requires a complex drive system.
A light wave interference method utilizing optical heterodyne techniques is known as a method of precisely measuring a moving speed of an object to be measured as well as the surface precision and the surface shape of the object.
Although an apparatus using the light wave interference method has an advantage in that physical information of the object can be obtained with high precision, there are a few measuring apparatuses of this type utilizing the optical heterodyne techniques.
Under these circumstances, there is no apparatus capable of simply measuring a gradient index or the like of an object to be measured with high precision.